Process for manufacturing a toothbrush

ABSTRACT

A process for manufacturing hollow handled toothbrushes is disclosed. The process includes; a) providing a toothbrush mold having a head portion, a base portion, a toothbrush cavity located between said head and end portions and having walls, an injection port for molten plastic, and a gas injection port, wherein the gas injection port is positioned in the end of the base portion of the mold so that gas is injected into the mold cavity substantially centrally thereof and in a direction parallel to the longitudinal axis of the mold; b) injecting a predetermined amount of a molten plastic into the mold cavity to partially fill the cavity; and c) injecting at least one gas through the gas injection port into the cavity to direct the molten plastic against the walls of the mold cavity.

FIELD OF THE INVENTION

[0001] This patent relates to an improved process for manufacturing atoothbrush. More specifically, the patent relates to a process formanufacturing hollow handled toothbrushes. The process utilizes a gaswhich is injected from a specific location in a toothbrush mold toassist in blowing a molten plastic therethrough. The injection of thegas in a specific location results in a more efficient process formanufacturing hollow handled toothbrushes.

DESCRIPTION OF THE PRIOR ART

[0002] Toothbrushes are typically manufactured using an injectionmolding process. The injection molding process is characterized byproviding a mold in the shape of the toothbrush and injecting moltenplastic through a hot channel nozzle into the mold. The toothbrush isthen cooled and ejected from the mold.

[0003] Toothbrushes may be difficult to handle for several reasons.Children may have difficulty handling toothbrushes due to the size ofthe child's hands. People with arthritis also sometimes have difficultyhandling toothbrushes, due to difficulty in flexing the joints in theirhands. Handicapped people may also have difficulty handlingtoothbrushes. There has been a recent trend to provide toothbrushes withrelatively large cross section handles to make handling the toothbrusheasier. Additionally, the larger cross section handles on thetoothbrushes may be better for the user from an ergonomics point ofview.

[0004] The manufacture of toothbrushes with larger cross section handleshas several drawbacks. Firstly, the toothbrush is more expensive due tothe use of more plastic to make the toothbrush. Secondly, the cost ofmanufacture is increased because the time to cool the toothbrushincreases. The increase in cooling time is due to the increased amountof hot plastic and the larger cross section of the toothbrush.Accordingly, there is a need for a more efficient process for makingtoothbrushes with larger cross section handles.

[0005] European Patent No. EP 668140 discloses the use of air assisttechnology to make toothbrushes with large cross section handles. In theprocess, molten plastic is injected near the base of the toothbrushhandle, perpendicular to the lengthwise direction of the toothbrush moldto partially fill the toothbrush mold. A hot needle is then insertedinto the molten plastic, also near the base of the toothbrush handle andalso perpendicular to the lengthwise direction of the toothbrush mold. Agas is blown through the needle to press the molten plastic against thewalls of the toothbrush mold. The disclosed process may be modified toinject a separate, less expensive polymer in place of the gas, in whichcase there is produced a solid handle in which the interior portions aremade from said separate polymer. In either case, the process iscompleted by injecting a small amount of plastic to close the hole leftby the needle.

[0006] European Patent No. EP 721832 also discloses the use of airassist technology to make toothbrushes with large cross section handles.In the disclosed process, molten plastic is injected near the head ofthe toothbrush mold, perpendicular to the lengthwise direction of thetoothbrush mold to partially fill the toothbrush mold. A hot needle isthen inserted into the molten plastic, also near the head of thetoothbrush, and also perpendicular to the lengthwise direction of thetoothbrush mold. A gas is then blown through the needle to press themolten plastic against the walls of the toothbrush mold. The processincludes transferring the hollow handled brush to a second mold andinjecting a separate plastic to fill the toothbrush handle.

[0007] We have found that gas injected just above the base of thetoothbrush handle, perpendicular to the lengthwise direction of thetoothbrush mold, as taught in European Patent No. EP 668140, may resultin inadequate hollowing of the toothbrush handle, i.e., some portions ofthe previously injected polymer remain as a “mound” near the gasinjection point and are not uniformly distributed against the walls ofthe mold. Additionally, it may be difficult to control the flow of thegas sufficiently to prevent hollowing of the toothbrush neck. Dependingon the inherent strength of the polymer being used, it may be desirableto avoid formation of neck portions which are hollow.

[0008] We have also found that the gas injected near the neck of thetoothbrush, perpendicular to the lengthwise direction of the toothbrushmold, as taught in European Patent No. EP 721832, may result ininadequate hollowing of the toothbrush handle. Additionally, the processis likely to lead to a hollowing of the toothbrush neck.

[0009] Therefore, despite the disclosure of the references, there is acontinuing need for a more efficient process for making toothbrusheswith large cross section handles.

SUMMARY OF THE INVENTION

[0010] The present invention provides a process for manufacturing atoothbrush including:

[0011] a) providing a toothbrush mold having a head portion, a baseportion, a toothbrush cavity located between said head portion and saidbase portion and having walls, an injection port for injecting moltenplastic, and a gas injection port for injecting a gas, wherein the gasinjection port is positioned in the end of the base portion of the moldso that gas is injected into the mold cavity substantially centrallythereof and in a direction parallel to the longitudinal axis of themold; b) injecting a predetermined amount of a molten plastic into thecavity to partially fill the mold cavity; and c) injecting a gas (or amixture of gases) through the gas injection port to direct, or force,the molten plastic into contact with the walls of the mold cavity,thereby forming a toothbrush having at least some portions which arehollow.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0012] The present invention utilizes an injection molding apparatus inthe manufacture of toothbrushes with hollow handles. Injection moldingmachines are well known in the art. Any such machine may be utilized inthe present invention. The present invention is herein described withrespect to FIGS. 1, 2, and 3 of the appended drawings. The key to thepresent invention is the location of the gas injection port. In order tocontrol the gas assist process of manufacturing toothbrushes at leastportions of which are hollow and in which the amount of plastic utilizedis minimized, the gas injection port (5) must be positioned in the baseend (3) of the toothbrush mold (1) such that gas flows into the moldcavity substantially centrally thereof.

[0013] The molten plastic injection port may be positioned anywhere onthe toothbrush mold, provided it communicates with the mold cavity.Preferably, however, molten plastic injection port (6) is located nearbase portion (3) of mold I as illustrated in FIG. 3. More preferably,molten plastic injection port (6) is located in the center of the end (3a) of base portion (3) as illustrated in FIGS. 1 and 2. Even morepreferably, the port through which the molten plastic is injected islocated concentrically around gas injection port (5) in end (3 a) ofbase portion (3) as illustrated in FIG. 1.

[0014] The toothbrush may be made of any well known plastic such as, butnot limited to, cellulose acetate propionate, nylon, polyethylene,polypropylene, polycarbonate, poly(ethylene terephthalate) or the like.Mixtures of two or more such plastics may be used. The plastic is heatedabove its melting point and injected through the molten plasticinjection port into toothbrush cavity (7) in toothbrush mold (1). Theamount of plastic injected will depend on the size of the toothbrush andthe desired thickness of the toothbrush walls. The amount of plasticinjected will be less than the total volume of the toothbrush cavity (7)in the toothbrush mold. The amount of plastic injected may range fromabout 10% to about 90%, preferably from about 20% to about 80% of thetotal volume of the toothbrush cavity (7), more preferably from about50% to about 80% of the total volume of cavity (7). It will beunderstood that lesser amounts of plastic may be used if desired.

[0015] Mold 1 includes a gate (not illustrated in the drawings)positioned at the point where molten plastic injection port (6) joinsthe body of the mold cavity. Various types of such gates and theirspecific orientation in the mold apparatus are well known in the art.Such gates allow the molten plastic to flow into the toothbrush mold.There are several types of known gates that may be utilized. One type ofgate that is useful is an edge gate. An edge gate is shaped like a slot,and has a length ranging from 1 mm to 10 mm, preferably from 5 mm to 7mm. Although useful, the edge gate may result in excess polymer flowinginto the toothbrush mold after the gas has been injected.

[0016] A second and preferred type of gate is a point gate. A point gateis circular in shape. The diameter of the gate may range from 1 mm to 10mm, preferably from 2 mm to 5 mm, even more preferably from 2 mm to 4mm. An annular gate is particularly preferred. The annular gate containsboth outer and inner tubes. The outer diameter of the outer tube of theannular gate may range from 1 mm to 10 mm, preferably from 2 mm to 5 mm,preferably from 2 mm to 4 mm. The diameter of the inner tube of theannular gate may range from about 0.1 mm to 9 mm, preferably from about0.5 mm to 5 mm Typically, the outer tube is utilized to inject themolten plastic, while the inner tube is utilized to inject the gas.

[0017] During the molding operation, at least one gas is injectedthrough gas injection port (5) into cavity (7) to push the moltenplastic towards the head portion (2) of the mold and to bring the moltenplastic into contact with walls (8) of cavity (7). The gas may be anyinert gas, including, but not limited to, air, carbon dioxide, nitrogen,and mixtures thereof The pressure at which the gas is injected may becontrolled, so that, if desired, the neck of the toothbrush is leftsolid. The pressure at which the gas is injected will depend on the typeof plastic utilized, the size of the cavity, and the desired thicknessof the toothbrush wall. Typically, the gas is injected at pressuresranging from about 340 kPa to about 4000 kPa, preferably from about 700kPa to about 2800 kPa, more preferably from about 1375 kPa to about 2400kPa. The pressure of the injected gas may be gradually increased duringthe gas injection stage, i.e., the initial injection gas pressure may berelatively low, but during the course of the gas injection, the pressureof the gas is increased. The pressure of the gas may be increaseduniformly or step-wise throughout the molding cycle.

[0018] The gas injection may begin after the molten plastic injection iscompleted. Alternatively, the gas injection may begin during the moltenplastic injection. For example, the gas injection may begin at any timeafter from about 10% to about 100%, preferably from about 30% to about90%, more preferably from about 50% to about 80% of the total moltenplastic injection has been completed.

[0019] The toothbrush is then cooled and released from the mold. Inorder to further reduce the toothbrush manufacturing cycle time, the gasutilized for the gas injection may be cooled, so as to begin the coolingprocess. Conventional cooling techniques, such as, but not limited tocool water lines around or in the toothbrush molds and blown air may beutilized to complete the cooling process.

[0020] The toothbrush may be finished by utilizing conventionaltechniques to secure bristles or bristle tufts to the head of thetoothbrush. The bristles may be stapled into the head of the toothbrushor fused into or onto the head of the toothbrush. The handle of thetoothbrush may be overmolded with materials such as thermoplasticelastomers or rubber. If desired, the overmolding process may be carriedout during the above described molding operation. Alternatively, thetoothbrush may be transferred to a separate mold for the overmoldingprocess.

[0021] Several examples are set forth below to further illustrate thenature of the invention and the manner of carrying it out. However, theinvention should not be considered as being limited to the detailsthereof.

Examples Of Reduction To Practice EXAMPLE 1 Gas Flow Perpendicular ToThe Lengthwise Direction Of The Toothbrush Mold

[0022] An injection molding machine designed such that the toothbrushmold was injected with molten plastic near the base portion of thetoothbrush mold, perpendicular to the lengthwise direction of thetoothbrush mold, and the gas was injected at the center of the end ofthe base portion of the toothbrush mold, parallel to the lengthwisedirection of the toothbrush mold, was used in this Example 1. Thelocation of the gas injection port in the center of the end portion ofthe mold helped ensure that the gas was injected into the central regionof the mold cavity. Molten polypropylene (9 grams/75 percent by volumeof the capacity of the mold cavity) was injected into the mold. Nitrogengas was then injected into the mold cavity for about 2 to 3 seconds. Thepressure of the gas at the start of the gas injection cycle was about1724 kPa/second. The gas injection pressure was uniformly increasedthroughout the gas injection cycle. The pressure of the gas at the endof the cycle was about 2070 kPa/second. The toothbrush was cooled andremoved from the mold.

[0023] Results:

[0024] The toothbrush was cut in half along the lengthwise direction ofthe handle. Although most of the handle was hollow; a mound of residualplastic was seen near the bottom of the handle where the molten plasticwas injected. Additionally, the thickness of the wall of the toothbrushwas not uniform. The cycle time to produce a solid toothbrush in thismold is typically 32 seconds. When this mold is used to produce atoothbrush having hollow portions as set forth in this Example 1, theprocess cycle time was reduced to about 24 seconds. The amount ofplastic needed to produce a solid toothbrush in this mold is typically12 grams. The amount of plastic needed to produce a hollow toothbrushaccording to this Example 1 was about 9 grams.

EXAMPLE 2 Gas Flow From The Center Of The Base End Of The ToothbrushMold

[0025] The injection molding machine described in Example 1 was modifiedsuch that the toothbrush mold was injected with molten plastic at thecenter of the end of the base portion of the toothbrush mold, parallelto the lengthwise direction of the toothbrush mold. The gas injectionport remained located in the center of the end of the base portion ofthe toothbrush mold. Molten polypropylene (9 grams/75 percent of thecapacity of the mold by volume) was injected into the mold. Nitrogen gasat an initial pressure of 1724 kPa was fed into the molten plastic for 3seconds, with the pressure at the end of the cycle being 2070kPa/second. The toothbrush was cooled and removed from the mold.

[0026] Results:

[0027] The toothbrush made according to Example 2 was cut in half alongthe lengthwise direction of the handle. The handle was hollow, and themound of residual plastic observed in the toothbrush of Example 1 waseliminated. The process of this Example 2 reduced the cycle time to 20seconds. The amount of plastic utilized to make the toothbrush utilizingthe apparatus and process of this Example 2 was 9 grams. The wall of thetoothbrush had a substantially uniform thickness.

We claim:
 1. A process for manufacturing a toothbrush comprising: a)providing a toothbrush mold having a head portion, a base portion, atoothbrush cavity located between said head portion and said baseportion and having walls, an injection port for injecting moltenplastic, and a gas injection port for injection at least one gas,wherein the gas injection port is positioned in the end of the baseportion of the mold so that gas is injected into the mold cavitysubstantially centrally thereof and in a direction parallel to thelongitudinal axis of the mold; b) injecting a predetermined amount of amolten plastic into the mold cavity to partially fill the cavity; and c)injecting said at least one gas through the gas injection port to directthe molten plastic into contact with the walls of the mold cavity. 2.The process of claim 1 wherein the injection port for the molten plasticis positioned near the base portion of the mold, perpendicular to thelengthwise direction of the toothbrush mold.
 3. The process of claim 1wherein the injection port for the molten plastic is positioned in thecenter of the end of the base portion of the mold.
 4. The process ofclaim 1 wherein the molten plastic is selected from the group consistingof cellulose acetate propionate, nylon, polyethylene, polypropylene,polycarbonate, and poly(ethylene terephthalate) and mixtures thereof. 5.The process of claim 1 wherein the amount of molten plastic injectedinto the mold ranges from about 10% to about 90% of the total volume ofthe mold cavity.
 6. The process of claim 1 wherein the amount of moltenplastic injected into the mold ranges from about 20% to about 80% of thetotal volume of the mold cavity.
 7. The process of claim 1 wherein thegas is selected from the group consisting of air, carbon dioxide,nitrogen, and mixtures thereof.
 8. The process of claim 7 wherein thegas is injected at a pressure ranging from about 340 kPa to about 4000kPa.
 9. The process of claim 8 wherein the gas is injected at a pressureranging from about 700 kPa to about 2800 kPa.
 10. The process of claim 9wherein the gas in injected at a pressure ranging from about 1375 kPa toabout 2400 kPa.